1. Field of the Invention
The present invention relates to a light-selective prism, a projection display apparatus using the same, and a method of manufacturing the light-selective prism.
2. Description of the Related Art
A cross dichroic prism is often used as the light-selective prism in a projection display apparatus that projects a color image. The cross dichroic prism has two different types of dichroic films that are arranged to cross in a substantially X shape. The cross dichroic prism has the function of a color combiner that combines three colored light components, red, green, and blue, together and emits a composite light beam in a specified direction.
FIG. 12 shows an essential part of a conventional projection display apparatus. The projection display apparatus includes three liquid crystal light valves 42, 44, and 46, a cross dichroic prism 48, and a projection lens 50. Red-reflecting films 48R and blue-reflecting films 48B are formed in a substantially X shape in the cross dichroic prism 48. The cross dichroic prism 48 combines the three colored light components, red, green, and blue, modulated by the three liquid crystal light valves 42, 44, and 46 and emits a composite light beam in the direction of the projection lens 50. The projection lens 50 focuses the composite light beam on a projection screen 52.
The cross dichroic prism 48 is generally manufactured by gluing the respective side faces of four columnar prisms having an identical size together. The red-reflecting films 48R are formed previously on predetermined side faces of two columnar prisms, so as to be located on the same plane when the four columnar prisms are glued together. In a similar manner, the blue-reflecting films 48B are formed previously on predetermined side faces of two columnar prisms.
The red-reflecting film 48R reflects only the red light component while transmitting the other colored light components. The blue-reflecting film 48B reflects only the blue light component while transmitting the other colored light components. Dielectric multi-layered films referred to as dichroic films are generally used for the reflecting films having such properties. These reflecting films bend the light components emitted from the liquid crystal light valves 42 and 46 by 90 degrees and thereby cause the light components to be uniformly directed to the projection lens 50. It is accordingly preferable that the reflecting films formed in a substantially X shape in the cross dichroic prism are respectively located on identical planes.
When the four columnar prisms of an identical size are glued together to constitute a cross dichroic prism, it is rather difficult to join the columnar prisms together with high accuracy in order to enable the red-reflecting films 48R and the blue-reflecting films 48B to be located respectively on identical planes. The poor accuracy causes both the red-reflecting films 48R and the blue-reflecting films 48B to have gaps and differences in level in the vicinity of the intersections of the respective reflecting films on the substantial center of the cross dichroic prism. The reflection properties and the transmission properties of the red-reflecting film 48R and the blue-reflecting film 48B are thus varied along the pass of the light beams passing through the vicinity of the intersections. This results in deteriorating properties of the cross dichroic prism 48. When such a cross dichroic prism is applied to the projection display apparatus, streaks due to the scattering of light in the cross dichroic prism are observed on the substantial center of a projected image. Another problem is that some color light reflected at center of the cross dichroic prism makes duplicate images on the center of the display, and the respective colored light components make displayed color component images of different sizes, accordingly.
The cross dichroic prism is also used to divide light into a plurality of light components. The problem of the deteriorating properties of the cross dichroic prism also arises in this case. A variety of light-selective prisms, in which light-selective films that selectively transmit or reflect light components are formed in a substantially X shape, other than the cross dichroic prism may be used as the optical elements. The problem of the deteriorating properties is commonly found in the variety of light-selective prisms.
The object of the present invention is thus to provide a technique that improves properties of a light-selective prism.
At least part of the above and the other related objects of the present invention is attained by a light-selective prism that has two different types of light-selective films crossing in a substantially X shape. The light-selective prism includes: four columnar prisms that are joined together on respective side faces thereof; and two light-selective films that are formed along the side faces of the four columnar prisms to cross in a substantially X shape, wherein one of the two light-selective films is continuously formed without separating at an intersection of the two light-selective films.
Since one of the two light-selective films is continuously formed without separating at an intersection of the two light-selective films, the properties of the continuous light-selective film is improved in the vicinity of the intersection of the two light-selective films that are arranged in a substantially X shape. This accordingly ensures an improvement of the properties of the resulting light-selective prism.
The one of the two light-selective films may be formed continuously on a curved surface.
The light-selective prism of this structure enables adjustment of the size of the image that is created by the light reflected from the light-selective film formed on the curved surface.
The light-selective film of the present invention may be manufactured according to one of the following methods. A first method manufactures a light-selective prism by joining four columnar prisms together on respective side faces thereof. The first method includes the steps of: (A) preparing two pairs of prisms by: i) providing two sets of two columnar prisms each having two joint side faces, ii) forming a first light-selective film on one of four joint side faces of the two columnar prisms of each set, and iii) gluing each set of two columnar prisms together across the first light-selective film; (B) evenly polishing joint faces of the two pairs of prisms across which the two pairs of prisms are to be joined together; (C) forming a second light-selective film over the even joint face of a selected one of the two pairs of prisms, the second light-selective film being continuous across a joint of the two columnar prisms in the selected pair; and (D) joining the two pairs of prisms together, so as to obtain a prism assembly.
The first method of the present invention causes the second light-selective film to be formed continuously across a joint of the two columnar prisms over one of the even joint surfaces, which belongs to a selected pair out of the two pairs of prisms. This arrangement effectively improves the properties of the second light-selective film.
In accordance with one preferable application of the first method, each set of two columnar prisms is a combination of a relatively long columnar prism and a relatively short columnar prism, and each pair of prisms are prepared in such a manner that one side face of the relatively long columnar prism, which is to contact with and glued to the relatively short columnar prism, has exposed portions on both longitudinal ends of the side face. In the step (D), the two pairs of prisms are joined together in such a manner that the exposed portions of each pair of prisms are in contact with a predetermined common reference surface.
This method enables the first light-selective films formed respectively on the two pairs of prisms to be positioned according to the predetermined reference plane and thereby to be present on the same plane.
The first method may further include the step of: cutting the light-selective prism out of the prism assembly obtained in the step (D).
This step enables a large number of light-selective prisms to be obtained from the prism assembly. The height of the light-selective prism can be set arbitrarily when the light-selective prism is cut out of the prism assembly. This gives a light-selective prism having a desired height.
In the first method, the step (B) may include the step of polishing at least one of the joint faces on which the second light-selective film is to be formed in the step (C), to a curved surface.
The light-selective prism manufactured in this manner enables regulation of the size of the image that is created by the light reflected from the light-selective film formed on the curved surface.
In the first method of the present invention, it is preferable that at least the joint side faces of the two columnar prisms of each pair of prisms, are polished.
This arrangement enables the first light-selective film to be formed on one of the polished joint side faces.
In this case, one possible arrangement further includes the step of: polishing an outer face of the prism assembly obtained in the step (D).
This enables the streaks and flaws on the glass surface, which may occur in each manufacturing step, to be removed on occasion.
Another possible arrangement further includes the step of: cutting a small prism out of the prism assembly obtained in the step (D), and polishing an outer face of the small prism.
A second method manufactures a light-selective prism by joining four columnar prisms together on respective side faces thereof. The second method includes the steps of: (A) providing a block of sheet glass having polished top and bottom faces; (B) forming a first light-selective film on one of the top and bottom faces of the sheet glass; (C) cutting the block of sheet glass with the first light-selective film formed thereon to obtain four columnar prisms, wherein two columnar prisms have faces on which the first light-selective film is formed and two columnar prisms do not have faces on which the first light-selective film is formed; (D) preparing two pairs of prisms by joining respective one of the two columnar prisms having the first light-selective film with respective one of the remaining two columnar prisms without the first light-selective film across the first light-selective film; (E) evenly polishing joint faces of the two pairs of prisms across which the two pairs of prisms are to be joined together; (F) forming a second light-selective film over the even joint face of a selected one of the two pairs of prisms, the second light-selective film being continuous across a joint of the two columnar prisms in the selected pair; and (G) joining the two pairs of prisms together, so as to obtain a prism assembly.
Like the first method discussed previously, the second method of the present invention causes the second light-selective film to be formed continuously across a joint of the two columnar prisms over one of the even joint surfaces, which belongs to a selected pair out of the two pairs of prisms. This arrangement effectively improves the properties of the second light-selective film. In the steps (A) through (C) of the second method, the four columnar prisms are prepared by dividing one block of sheet glass into four pieces. This enables the four columnar prisms to have a uniform refractive index. The uniform refractive index effectively prevents a variation in size of the displayed image due to a difference between the optical paths of the light components transmitting through the resulting light-selective prism.
A cross dichroic prism, which is one embodiment of the light-selective prism according to the present invention, may be applied to a projection display apparatus. The present invention is accordingly directed to a projection display apparatus that projects and displays an image. The projection display apparatus includes: an illumination system that emits illumination light; a color separator that divides the illumination light into three colored light components; three light modulator that respectively modulate the three colored light components based on given image signals respectively; a cross dichroic prism that combines the three colored light components modulated by the three light modulator to a composite light beam; and a projection optical system that projects the composite light beam combined by the cross dichroic prism. The cross dichroic prism comprising: four columnar prisms that are joined together on respective side faces thereof; and two dichroic films that are formed along the side faces of the four columnar prisms to cross in a substantially X shape, wherein one of the two dichroic films is continuously formed without separating at an intersection of the two dichroic films.
This projection display apparatus generates the composite light beam using the cross dichroic prism according to the present invention. This structure relieves the problem that streaks due to the scattering of light in the cross dichroic prism are observed on the substantial center of a projected image.